Tornado Debris Signature (TDS)

Tornado Debris Signature (TDS)

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A tornado hit parts of Okeechobee and Indian River Counties in Florida this morning. We know the tornado occurred because of a powerful radar signature known as a “Tornado Debris Signature.” I capitalize out of respect for its life-saving utility.

Nowadays, many National Weather Service offices include radar-confirmed language in tornado warnings in real time.

This is thanks to an upgrade all National Weather Service radars underwent about five years ago, called dual-polarization.

Meteorologists can use this technology to confirm a tornado without a human even seeing it! How’s that for government money well spent?

So, let’s dive into the radar meteorology lesson.

TOP LEFT: Reflectivity (Z). This is your typical radar view. Heavy rain, hail and in this case, debris cause brighter colors. The “debris ball” is underneath the crosshairs.

TOP RIGHT: Velocity (V). Due to a scientific limitation, the radar changes a setting (pulse repetition frequency) between the low-level reflectivity and velocity scans. So, even though the velocity couplet is offset to the east by a radial, no concern – the velocity scan was likely a few dozen seconds later than the other images.

BOTTOM LEFT: Correlation Coefficient (CC). This identifies the similarity of radar targets. The clear depression (blue circle) is because the radar is seeing rain drops, sticks and other debris the tornado has picked up off the ground. Plain rain would generate very similar values and thus be close to a solid color on this panel.

BOTTOM RIGHT: Differential Reflectivity (ZDR). This shows meteorologists how targets compare in the horizontal and vertical. A value at or near zero means the target is round. Guess what? Tumbling debris looks round to the radar. Thus, the zero value.

It’s important that all of these criteria are collocated. Near-zero differential reflectivity isn’t required to confirm a TDS, but it adds confidence. Several volume scans (radar looped over a period of time) also helps immensely since most tornadoes move in a quasi-linear fashion. Continuity in the vertical is also helpful, though only the stronger tornadoes send debris high enough for upper-level scans to see.

Now, the disclaimer here is that many tornadoes occur without debris signatures. So it’s not always this easy. If there’s nothing for the tornado to hit, there’s no debris for it to spread into the sky. The debris-churning tornado must also be within a reasonable range of the radar site for the signature to have a chance at appearing.

Additionally, a tornado debris signature can only occur after a tornado has spun up debris. So, from a warning perspective, it doesn’t help with early warning. But it can help with meteorologist confidence and thus the severity of language used during an ongoing event, and that saves lives.


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